The present invention relates to methods for producing hydrogen from hydrocarbon fuels and reactors for carrying out the methods; and more particularly to methods, apparatus, and catalysts for conducting water gas shift reactions on a reactant stream of hydrocarbon fuels having been previously reformed by partial oxidation, steam reforming, or both.
Reforming of hydrocarbon fuels to make hydrogen is well known in the art. Conventionally, hydrocarbons are reformed predominately in large-scale industrial facilities providing hydrogen for bulk storage and redistribution, or producing hydrogen as an on-line, upstream reagent for another large-scale chemical process. For the most part, these prior processes operate continuously and at steady-state conditions.
More recently, however, a strong interest has developed in providing hydrocarbon-reforming reactors integrated with an end use of the hydrogen. Also, there is a strong interest to develop a low-cost, small-scale source for hydrogen that can replace the need for storing hydrogen gas on site or on board. More particularly, a great interest has developed in providing reactors for producing hydrogen, which can be integrated with a fuel cell which uses hydrogen as a fuel source to generate electricity. Such hydrogen generator/fuel cell systems are being pursued for stationary uses such as providing electrical power to a stationary facility (home or business), for portable electric power uses, and for transportation.
There are many technical requirements for reactors used in such applications, which are not required of traditional large or small-scale hydrogen generating reactors. For example, it is of particular interest to have such a system where the fuel cell can provide xe2x80x9cpower on demand.xe2x80x9d Hence, hydrogen must be produced at required variable levels on demand. In other words, the hydrogen producing reactors must be sufficiently dynamic to follow the load. It is also of interest that such systems perform well upon start up and shutdown cycling. In particular, it is desirable to have these integrated systems be stable through repeated on-off cycling, including being ready to come back on-line in a relatively short time after periods of non-use.
Another marked difference between proposed integrated systems and traditional reactors is that there must be sufficient processing in the integrated system itself, and of the hydrocarbon feed stock so as to not only give a yield of hydrogen sufficient to meet the demand, but also to minimize byproducts of reaction including contaminants. In large-scale reactor systems, which produce enormous volumes and run continuously, space, weight, and cost of auxiliary systems is not so critical as in the integrated, smaller-scale reformers, especially those proposed for portable power or transportation applications. For example, carbon monoxide may be considered an undesirable reaction product on board a fuel cell powered automobile. However, in a steady state conventional process, the carbon monoxide can easily be handled by auxiliary separation systems, and may in fact be welcomed for its use in a synthesis gas to make acetic acid, dimethyl ether and alcohols.
In short, the challenge for the smaller-scale, dynamic, integrated processors is the idea that what goes in the reformer must come out at the same end as the desired hydrogen gas. Accordingly, processing has to be more complete and more efficient, while cost effective, lightweight, and durable. The processing must be sufficient to reduce or eliminate species in the product gas which are harmful to the end use (for example, fuel cells) or other down stream components.
Another challenge exists for the proposed integrated systems with respect to the hydrocarbon feed stock. To be of maximum benefit, the proposed integrated systems should be able to use existing infrastructure fuels such as gasoline or diesel fuels. These fuels were not designed as a feed stock for generating hydrogen. Because of this, integrated systems are challenged to be able to handle the wide variety of hydrocarbons in the feed stock. For example, certain reforming byproducts such as olefins, benzene, methyl amide, and higher molecular weight aromatics can cause harm to catalysts used in reforming or purifying steps and may harm the fuel cell itself. Impurities in these fuels such as sulfur and chlorine can also be harmful to reactor catalysts and to the fuel cell.
It is also important to note that a natural byproduct of hydrocarbon reforming is carbon monoxide. Carbon monoxide can poison proton exchange membrane (xe2x80x9cPEMxe2x80x9d) fuel cells, even at very low concentrations, e.g., less than 100 PPM. This poses a problem for an integrated reactor system that is not faced by traditional reforming processes where significant carbon monoxide concentrations are either a useful co-product, or can be separated from the product gas without undue burden on the system economics as a whole.
Also, as noted above, integrated systems proposed to date are expected to transfer the total of the reformate to a fuel cell. Accordingly, techniques which separate carbon monoxide from hydrogen, such as pressure swing adsorption (xe2x80x9cPSAxe2x80x9d) or hydrogen permeable membrane separation, have the deficit of having to provide an alternate means for disposal or storage of the carbon monoxide. Both of the aforementioned techniques also suffer in efficiency as neither converts the carbon monoxide (in the presence of water) to maximize hydrogen production. PSA also suffers from high cost and space requirements. Most notably, PSA presents a likely unacceptable parasitic power burden for portable power or transportation applications. At the same time, hydrogen permeable membranes are expensive, are sensitive to fouling from impurities in the reformate, and reduce the total volume of hydrogen provided to the fuel cell.
At the levels of carbon monoxide present in the reformate stream after partial oxidation, steam reforming or ATR (for example, less than or equal to about 20% carbon monoxide), catalytic techniques such as preferential oxidation (xe2x80x9cPROXxe2x80x9d) or selective methanation are not efficient options. Although it should be noted that PROX and selective methanation may both be appropriate as a secondary, or clean up, process at suitably low carbon monoxide levels. For example, PROX appears to be suitable for oxidizing carbon monoxide at residuals of 20,000 PPM or less.
On the other hand, implementing and using water gas shift reactions does not present the impairments of the above-discussed techniques. Hence use of a water gas shift reactor is highly preferred.
Reformation of hydrocarbons (for example, alcohols, methane, propane, butane, pentane, hexane, and various other gaseous and liquid petroleum fractions saturated and unsaturated, cyclic compounds, aromatic compounds, etc.) may be subjected to some form of partial oxidation to create a reformate enriched in hydrogen. This partial oxidation can be accomplished by a flame-type gas-phase reaction or can be catalytically promoted, for example by a nickel-containing catalyst. Water in the form of steam may be added to prevent coking of the hydrocarbons during oxidation. Reformate composition varies widely with the type of hydrocarbon fuel or feed stock and with the efficacy of the particular partial oxidation process employed. However, reformate generated in this way generally includes varying amounts of carbon monoxide, carbon dioxide, water, nitrogen, trace amounts of hydrogen sulfide, and in the case of partial oxidation, ammonia. Beyond these chemicals, the remainder of the reformate being methane, ethane and depending on the fuel, other higher molecular weight hydrocarbons including: unsaturated and aromatic species; oxygenated species such as ethers, esthers, alcohols, aldehydes, etc.
Steam reforming may also be used to produce hydrogen by promoting the following reaction Equation 1, with a catalyst such as a nickel supported on a refractory material:
CnHm+nH2Oxe2x86x92nCO+(m/2+n)H2
where n=an integer of 1 or greater and m=an integer of 2 or greater, for example, Equation 2:
CH4+H2OCO+3H2
Again, reformate composition resulting from the steam reforming process varies widely with the type of hydrocarbon fuel or feed stock and with the efficacy of the particular catalyst and process parameters employed. Again, however, the reformate generated in this way generally includes (in addition to hydrogen) varying amounts of carbon monoxide, carbon dioxide, and water, with the remainder being methane, ethane and potentially higher molecular weight hydrocarbons including unsaturated and aromatic species, ethers, esthers, alcohols, aldehydes, etc. Depending on the sulfur content of the fuel used, the reformate can include trace amounts of hydrogen sulfide.
It has been proposed to use partial oxidation in combination with steam reforming with the former being upstream of and providing methane-rich reactant feed to the steam reforming step, for example see e.g. WO 98/08771, published Mar. 5, 1998, assigned to Applicant. The coupling of an exothermic partial oxidation reaction with an endothermic steam reforming reaction is sometimes referred to as xe2x80x9cautothermal reforming,xe2x80x9d or xe2x80x9cATR.xe2x80x9d
Carbon monoxide produced from either partial oxidation or steam reforming can react with water (present from the prior oxidation process or intentionally added to the system) according to the following water-gas-shift (WGS) reaction of Equation 3 to generate more hydrogen:
CO+H2OCO2+H2
However, the extent of the WGS reaction is limited by equilibrium concerns. At the elevated temperatures required for steam reformation (typically between 650xc2x0 C.-980xc2x0 C.), the purity or yield of hydrogen is limited by the equilibrium. Hence, it has been proposed to subject the feed stream emanating from the steam reforming step to one or more catalytically promoted shift steps. As disclosed in WO 98/08771, such an integrated system provides for a high temperature shift reaction promoted by an iron-containing catalyst, followed by a low temperature shift process promoted by a copper-containing catalyst. In that system, the high temperature shift process takes advantage of relatively higher kinetics at the higher temperature, while sacrificing desired equilibrium. The relatively lower temperature shift process can then take advantage of a more favorable WGS equilibrium to provide a higher yield of hydrogen, while it benefits from the preliminary level of conversion in the high temperature shift.
Problems exist with conventional WGS catalysts, particularly the copper-based and zinc-based catalysts used for so-called xe2x80x9clow temperature shift,xe2x80x9d in an integrated fuel reformer. These catalysts are adversely affected by many of the common reformate species discussed above such as unsaturated and aromatic compounds. These catalysts are also adversely affected by contaminants in the feed stock such as sulfur and halogen compounds, all of which can routinely exist in the reaction gas stream as it enters the xe2x80x9clow temperaturexe2x80x9d shift catalyst down stream of the aforementioned reformer processes in an integrated system.
During startup and shutdown of a dynamic reactor, the frailties of these catalysts are even more pronounced. At start up, the upstream reactors are not up to peak efficiency temperatures and hence there are higher concentrations of (and perhaps more) harmful reactant species produced, such as unsaturated and aromatic compounds, which can poison these catalysts. After shutdown, steam in the system can condense on the catalyst. This condensed steam deactivates these conventional catalysts prematurely by permitting the copper and zinc to mobilize in the liquid phase condensate (i.e., water).
Also, these conventional copper and zinc catalysts must be reduced (usually in situ) by a controlled atmosphere artificially being fed into the reactor to control the rate of reduction to avoid excessive heat which can spoil the catalyst. Once reduced, further burdensome care during manufacture and maintenance of the reactor is needed to avoid contact with oxygen in the air, because these catalysts spontaneously oxidize in the presence of air and release heat during the process. When this happens, the catalyst needs to be reduced again. The subsequent reduction of the catalyst is also exothermic. Ultimately, the heat from exothermic reduction and/or oxidation reduces the catalyst life.
The present invention addresses the above mentioned deficiencies in the art and provides additional advantages as will be disclosed more fully below.
The present invention is directed to processes and reactors for converting carbon monoxide and steam in a reformate stream into carbon dioxide and hydrogen while employing an improved catalyst. The process includes generating a reformate by reacting a hydrocarbon fuel via partial oxidation, steam reforming, or both. The reformate is then reacted in the presence of a platinum group metal selected from the group consisting of platinum, palladium, iridium, osmium, rhodium or mixtures thereof. According to another aspect of the invention, the platinum group metal is supported on a material selected from the group consisting of an oxide of zirconium, titanium and mixtures thereof. The preferable catalyst and support is Pt/ZrO2. According to another aspect of the invention, a water gas shift reaction can be accomplished in a reformate over a wide range of temperatures (for example, between about 200xc2x0 C. to about 650xc2x0 C.) using a single shift catalyst.
According to another aspect of the invention, an advantageous process enhancement includes the step of introducing a predetermined amount of oxygen into the reformate for a desired period of time, oxidizing hydrocarbons, carbon monoxide and hydrogen, in the presence of the catalyst to generate heat to produce a desired temperature in the catalyst. This takes advantage of the heating value of the reformate at start up, when the reformate may not yet be acceptably pure for delivery to the fuel cell.
A reactor according to the invention includes a first reactor section configured to produce reformate by a process selected from the group of partial oxidation, steam reforming, or a combination thereof. A second reactor section is put in communication with the first reactor section so as to receive the reformate. A catalyst is located in the second reactor section. The catalyst comprises a platinum group metal selected from the group consisting of platinum, palladium, iridium, osmium, rhodium and mixtures thereof, and a support material, for the platinum group metal, selected from the group consisting of an oxide of zirconium, titanium and mixtures thereof.
Use of this process and apparatus provides a number of advantages over prior art water gas shift catalysts. For example, the catalyst of the invention can be operated at higher temperatures than conventional xe2x80x9chigh temperaturexe2x80x9d shift catalysts containing iron. Catalysts according to the invention are also expected to have a higher activity than iron-containing catalysts. Also, as noted above, commercial Cu/ZnO catalysts or so called xe2x80x9clow temperature shiftxe2x80x9d catalysts can undergo exothermic oxidation and reduction reactions, which in turn, can cause the catalyst temperature to rise to undesirable levels. This is not the case for the catalyst of the invention, because it can be used at relatively lower metal loading due to its activity. The low metal content in the Pt/ZrO2 catalyst, for example, minimizes any temperature rise.
Also it is believed that a strong metal support interaction (xe2x80x9cSMSIxe2x80x9d) occurs between the platinum group metals (xe2x80x9cPGM""sxe2x80x9d) and the supports disclosed which aids in structural integrity. Cu/ZnO catalysts are not known to have the added integrity provided by an SMSI interaction.
Apart from the lack of an SMSI structural attribute, Cu/ZnO catalysts are also susceptible to sintering promoted by chlorine and other halogens. The Pt/ZrO2 catalyst should be resistant to this form of deactivation because the melting point of PtCl2 is much higher than the melting point of CuCl2.
According to another broad aspect of the invention, the metal catalyst is deployed on the support without the use a halide salt. The preparation method eliminates the possibility of any leftover halogen on the catalyst. This will prevent any possible problems to the WGS catalyst or any downstream processes caused by halogens. Platinum on ZrO2 (as discussed below) has been tested to date, but other Platinum Group Metals are also expected to work. Transition metals may also benefit from the ZrO2 support and result in a more cost-efficient solution (e.g., Cu/ZrO2).
Notably, the catalyst of the invention does not need a special controlled reducing atmosphere for initial reduction as do the Cu/Zn catalysts. The catalysts of the invention can be reduced (if needed) by the constituents in the reformate stream during normal operation.
It is also believed that other supports capable of producing SMSI effects may be good supports for WGS catalysts according to the invention (e.g., TiO2, etc.).